1. Field of the Invention
The present invention relates to a method for judging whether a semiconductor wafer is a non-defective wafer or not by inspecting the surface of a semiconductor wafer by a laser scattering method, the surface on which a device pattern has not yet been formed. More specifically, the present invention relates to a method for inspecting a semiconductor wafer by using a haze signal from an inspection apparatus using a laser scattering method.
2. Description of the Related Art
As a microelectronic device structure has recently become finer, dealing with particle induced defects and other defects on the surface of a semiconductor wafer on which a device pattern has not yet been formed has become extremely important. The particle induced defects and other defects near the surface of the semiconductor wafer greatly affect the quality of a semiconductor device. Therefore, to improve the quality of the device, it is necessary to evaluate the particle induced defects and other defects near the surface of the semiconductor wafer which becomes a substrate of the device and grasp the actual conditions of the particle induced defects and other defects.
Incidentally, semiconductor wafer manufacturers and device manufacturers conduct an inspection for evaluating these particle induced defects and other defects near the surface of the semiconductor wafer by using an apparatus (for example, refer to Patent Document 1) using scattered light generated when light is made to enter an object to be measured. Based on the number of LPDs (light point defects) on the wafer surface detected by an inspection apparatus using such a laser scattering method, a judgment is generally made as to whether the wafer is a non-defective wafer or a defective wafer. The inspection apparatus detecting the LPDs by the laser scattering method illuminates the wafer with a laser from above, condenses the scattered light from the LPDs with a condenser, converts the scattered light into an electrical signal with a photoelectric conversion element, and detects the electrical signal. However, since a requirement for an LPD detection lower limit is now below 65 nm and the intensity of scattering from the LPDs themselves is decreased, the judgment results obtained by the wafer manufacturer and the judgment results obtained by the device manufacturer do not necessarily coincide with each other. As a result, there are more and more cases in which the wafer judged to be a non-defective wafer by the wafer manufacturer is judged to be a defective wafer by the device manufacturer.
On the other hand, the scatter components from the wafer surface in such an inspection apparatus using a laser scattering method include a haze signal, which is a background signal from which a signal from the LPD is removed. The haze is the sum of the haze caused by microroughness in a wafer plane and the haze caused by light interference of a transparent film. As a result, as will be described later, the haze is different from the actual LPD. However, a requirement for an LPD detection lower limit is now below 65 nm, and a difference between the scattering intensity from the LPDs in the wafer plane and the scattering intensity from the haze is very small. Therefore, even in a wafer, which is judged to be non-defective in the inspection conducted by the wafer manufacturer, a spot in which the haze is partially high may be erroneously recognized as an LPD depending on the stability of the inspection apparatus of the device manufacturer. It is for this reason that, even when the area recognized as including the LPD by the inspection apparatus is observed again with an SEM (scanning electron microscope), particle induced defects or other defects which become a problem in the device production process are often not found actually in that area. Such a disparity between the judgment results obtained by the wafer manufacturer and the judgment results obtained by the device manufacturer in the inspection of the LPDs conducted by using the laser scattering method often causes a problem such as a temporary suspension of the production line.
The inventors have found out that a sensory method of making a visual inspection of each haze map on which the haze signals are shown can prevent such a problem. However, such an inspection method by which a visual inspection of each haze map is made by the human eye results in delays in the production line. Therefore, as a cause of such a disparity between the judgment results obtained by the wafer manufacturer and the judgment results obtained by the device manufacturer based on the number of LPDs, the inventors have further focused attention on the stability of the inspection apparatus, in particular, the difference in the S/N ratio (signal/noise ratio) between the inspection apparatuses. That is, even when both the semiconductor wafer manufacturer and the device manufacturer judge whether a semiconductor wafer is a non-defective wafer or a defective wafer by the inspection apparatus using the laser scattering method, there may be a difference in judgment results when the inspection apparatuses have different numbers or are of different models. In addition, even when identical inspection apparatuses are used, there is a difference in judgment results due to the difference in the measurement mode or throughput settings or the difference in detectors provided in the identical inspection apparatuses. Here, S (signal) is scattered light, and N (noise) is incident light or the like.
In the inspection apparatus using the laser scattering method, the size of a particle induced defect or other defect as an LPD is determined based on the intensity of scattering from a PSL (polystyrene latex) standard particle. However, the apparatuses of different models have different laser illumination systems or scattered light detection systems, and therefore have different scattering behaviors in particle induced defects or other defects and different detection capabilities. As a result, a pattern on a haze map, the pattern recognized as haze in an apparatus, sometimes exceeds a threshold of the scattering intensity corresponding to an LPD size lower limit, and is erroneously detected as a pseudo LPD by another apparatus. The same erroneous detection can occur due to the difference in the measurement mode or detectors between the inspection apparatuses. Furthermore, the same erroneous detection can occur due to the difference in throughput settings. This is because, in a common inspection apparatus, the throughput is enhanced by making the beam spot diameter greater than a maximum sensitivity by sacrificing the sensitivity and reducing the number of scanning operations performed on the wafer. As a result, since even identical apparatuses use different reference calibration tables for the intensity of scattering from the PSL standard particle, the reference calibration tables provided one for each throughput, the same erroneous detection occurs due to a subtle difference in the scattering intensity in LPDs which are equal to or smaller than 65 nm, the LPDs in which the difference between S (signal) and N (noise) has become almost zero.
Such erroneous detection occurs not only between the inspection apparatuses using the laser scattering method, but also can occur even when, for example, the wafer manufacturer conducts an inspection with an inspection apparatus using the laser scattering method and the device manufacturer conducts an inspection with a bright-field inspection apparatus. This is because the bright-field apparatus uses light as in the inspection apparatus using the dark-field laser scattering method.
Patent Document 1    Japanese Examined Patent Application Publication No. 63-140904 (Claims)
Therefore, the inventors have focused attention on the haze map generated from the haze signals which are background signals in the LPD measurement performed with the inspection apparatus using the laser scattering method. As a result, for automation of an inspection using the haze signals, the inventors have found a method for judging whether a semiconductor wafer is a non-defective wafer or a defective wafer based on the relationship between the in-plane standard deviation and the in-plane average value of the haze signals, and solved the above problems.